sysmobts: Transmit the UI idle frame as a LAPDm Command
sysmobts: Transmit the UI idle frame as a LAPDm Command

When transmitting an idle frame the BTS should transmit it as a
command and not a response. This is unbreaking the mobile application
of osmocomBB.

osmocom/osmo-bts,geosphere/osmo-bts,geosphere/osmo-bts,shimaore/osmo-bts,osmocom/osmo-bts,telenoobie/osmo-bts,geosphere/osmo-bts,shimaore/osmo-bts,osmocom/osmo-bts,telenoobie/osmo-bts,telenoobie/osmo-bts,telenoobie/osmo-bts,geosphere/osmo-bts,shimaore/osmo-bts,shimaore/osmo-bts